Excessive steaming

"I have a problem with my 1987

"I have a problem with my 1987 272 Formula. I have excessive steam coming from my port side mercruiser 350. Here is the background.
alpha one outdrives, new impeller, new engine circulating pump, new exhaust manifolds and risers, new thermostat, boat is in a slip 4 months a year. The boat was used in a freshwater lake until I bought it 5 years ago. I installed perko flush kits immediately and use them everytime I go back to the dock. The engines are raw water cooled. I changed the plugs and none of them had any rust on them. The engine runs great except when idling down to the bay, i have to watch the temperature because sometimes it climbs way above 155, sometimes to almost 200. I immediately put the drive in neutrel and rev the engine and the temp drops down again. The temperature only rises at low idle. It is fine when cruising. But there is the constant steam coming from the silent thunder exhaust. (Basically a through hull exhaust.) Any ideas of help would be appreciated."

""new impeller...The engin

""new impeller...The engine runs great except when idling down to the bay...temperature because sometimes it climbs way above 155, sometimes to almost 200. I immediately put the drive in neutrel and rev the engine and the temp drops down again."

That sounds like a worn or improperly installed impeller."

"I don't think it is the i

"I don't think it is the impeller. I've been having this problem for two seasons. When I pulled out the impeller it looked great. I replaced the housing and the base as usual. I usually change them on both drives each season. I'm not a real mechanic, but I have done all the maintenance on the boat for five years. Not that I couldn't have messed up. When I run the boat in the driveway, there is plenty of water coming out of the exhaust at idle."

"are you using muffs in the dr

"are you using muffs in the drive way, if so the water pressure can bypass a bad impeller and make it seem good, use a clean garbage can full of water and see what happens."

"I had the problem for the las

"I had the problem for the last two years. When it first started happening, i changed the impeller because I thought it might be a bad impeller. It wasn't. I also changed the circulating pump, the thermostat and the manifold and risers. The engine continued to steam and overheat when idling in gear down the river. I changed the impeller, the housing, and the base about a month ago. The other impeller came out looking great. I also changed the impeller on the port side outdrive. That one looked good also. No steam from the other engine. Three impellers later and it still steams. I don't think its the impeller. Thanks for the input. This problem is driving me nuts."

"Steam usually means that you

"Steam usually means that you are getting some raw water through the system, but not enough to fully cool the motor. So, my guess is to check the intake hose, going from the seacock to the water pump. The hose is put under high vacuum pressure from the water pump when you turn the engine over. The hose may be partially collapsed, and is restricting the water flow into the pump. The hose is usually under the motor, so you have to get down there to check."

Thanks. I'll disconnect t

Thanks. I'll disconnect the hose at the transom (alpha one) and check the flow from there. I have checked the power steering cooler in the past but not lately. Thanks for the input.